Dr. Mark Beatty, health officer with Snohomish County, left, and Dr. Kathy Lofy, health officer with the Washington State Department of Health, talk with reporters Wednesday about the four new confirmed cases of measles in King, Snohomish and Pierce counties. (Ellen M. Banner / The Seattle Times)

Health officials have confirmed a Western Washington measles outbreak, with four new cases reported among people who spent time at Seattle-Tacoma International Airport and other sites throughout the Puget Sound region.

Two of the new cases involve King County women, the others a high-school student in Snohomish County and a Pierce County man, the Washington State Department of Health said Wednesday.

The outbreak has led Issaquah High School to close Thursday to verify the immunization records of all staff, after one King County woman went to the school not knowing she had measles. Health officials are also barring anyone who can’t document vaccination from the school until 21 days after possible exposure. About 1.8 percent of Issaquah students had measles-vaccine exemptions in 2017-18, according to state health-department data.

While 90 percent of kindergartners and 96 percent of sixth graders in Washington are fully vaccinated against measles, state health officer Dr. Kathy Lofy, with the Washington State Department of Health, warns that outbreaks are likely to continue here, mirroring the rest of the country.

“Given that measles is so contagious, I think it’s likely that we will see more measles cases here in Washington,” Lofy said, citing the public locations people recently infected with the disease had visited.

Washington has seen about 9 percent of the 839 measles cases reported in the United States this year, which prompted state lawmakers to ban exemptions to the measles, mumps and rubella vaccine except for religious or medical reasons. The outbreak has brought the highest number of cases nationally since 1994 and since measles was declared eliminated in 2000. Twenty-three states have been affected.

Snohomish County public-health officer Dr. Mark Beatty said the Snohomish County student who contracted measles attends North Creek High School in Bothell. Health officials are working with the school to send letters to students and staff without vaccination records, notifying them that they are barred from North Creek for 21 days.

At North Creek, 1.7 percent of students were unvaccinated during the 2017-18 school year, according to the state Department of Health.

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One of the five people most recently diagnosed has been hospitalized, Lofy said. One person was fully immunized and one was not immunized, she said. The immunization status of the other three people hasn’t been determined.

While people who have been immunized can still contract the disease, the measles, mumps and rubella vaccine is 97 percent effective after two doses, Lofy said. And in the unlikely event that a person who is vaccinated does contract measles, the disease is typically milder, she said.

“The bottom line is everybody just needs to check to see if their immunizations are up to date,” Lofy said.

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Health officials haven’t determined where the five people were exposed, but all had been at Sea-Tac airport during the period they were infected. Officials hope to have more details this week on when the five people were at the airport, and they are testing to see if all patients contracted measles from the same source.

The total number of known measles cases in Washington now stands at 77, with the vast majority reported in Clark County among unvaccinated children. No new cases have been reported there in recent weeks, according to the state health department.

Measles, which can cause fever, rash and red, watery eyes, is highly contagious and spreads quickly through the air after a cough or sneeze. Symptoms may appear starting from seven days after the first exposure to 21 days after the last exposure, according to the state health department. A rash typically appears 10 to 12 days after exposure.

People who have not been vaccinated or who have not had measles before may be at risk. Those who believe they may have been exposed should call a health-care provider. To prevent spreading measles, patients should call to discuss being evaluated instead of just walking in.

Health officials in the three county departments said they are working to determine all the sites each person went while contagious. The virus can remain in the air for up to two hours after the infected person leaves, so the lists of exposure times also include the additional two-hour time frames.

Some parents choose not to vaccinate their children for a number of reasons, including the belief vaccines can cause autism, which has been disproved by studies by the Centers for Disease Control and Prevention.

Beginning in July, Washington parents will no longer be able to claim personal or philosophical exemptions from the measles, mumps and rubella vaccine for their children. Parents can still claim medical or religious exemptions under the new law signed by Gov. Jay Inslee this month.